JPH05331523A - Method for refining molten steel for bearing steel - Google Patents

Abstract

PURPOSE:To obtain a steel material for bearing having little oxygen content and Al2O3 series inclusions and excellent rolling fatigue service life characteristic by further executing deoxidize-refining with Al after executing pre-deoxidize- refining to a high carbon molten steel by a vacuum refining apparatus. CONSTITUTION:The molten high carbon steel containing 0.80-1.20wt.% C is reacted between remaining oxygen and C in the molten steel as CO gas by the vacuum degassing refining apparatus, to execute the pre-deoxidize-refining treatment. In the molten steel reducing [O] by this pre-deoxidation, by adding Al, Al and [O] in the molten steel are reacted to produce the steel for bearing having extremely little quantity of the Al2O3 series inclusions of >=20mum grain diameter, such as <1 piece/100mm<2> and excellent rolling fatigue service life.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is a proposal for refining low-oxygen molten steel, particularly for refining a bearing steel which has few inclusions and excellent rolling fatigue life characteristics.

[0002]

2. Description of the Related Art It is known that the rolling fatigue life of bearing steel is strongly influenced by the total [O] concentration in the steel, as shown in FIG. Even 20
It is also known that the presence of oxide-based (Al ₂ O ₃ -based) inclusions of μm or more significantly shortens the life. In general, bearing steel is, as disclosed in JP-A-62-294150, a molten steel in a refining vessel, an inert gas is supplied from the bottom of the refining vessel to stir the molten steel while stirring the molten steel. And oxidative refining by spraying an oxidizing gas from the top as needed, then removing the slag and stirring the gas while adjusting the temperature of the molten steel by electrode heating in the presence of basic slag. It is manufactured by a refining method in which reduction refining is performed while performing vacuum degassing treatment thereafter. In such a known refining method, Al is added for deoxidation at the initial stage of reduction refining in which the [O] concentration in steel is relatively high.
Al ₂ O ₃ -based inclusions of 20 μm or more tend to remain in the molten steel after refining. In this way, if 20 μm or more of Al ₂ O ₃ type inclusions remain in the product bearing in the steel, the rolling fatigue life of this steel will be significantly reduced.

For high-grade bearings, the concentration of [O] in steel is lowered.
A method for refining steel is disclosed in JP-A-1-252722.
The disclosed method for treating high-grade bearing steel by the RH method is
is there. The disclosure in Japanese Patent Laid-Open No. 1-252722 discloses a ladle lie.
Specified ninging material, limiting the amount of oxygen blowing for heating, heating agent
Adjusting the reflux time after loading and synthesizing flux into the ladle before RH treatment
Reduce the oxidation loss of [Al] by taking measures such as adding Lux.
It is a technology that aims to reduce the total amount of [O] in steel.
It However, in the case of this known technique, the [O] concentration
Since Al is added in the state of high crude molten steel to deoxidize it,
As in the case of the conventional technique, Al of 20 μm or more is contained in steel. ₂O₃
There is a problem that system inclusions are likely to remain.

[0004]

As can be seen from the above description of each of the prior arts, the rolling fatigue life of bearing steel is proportional to the total [O] concentration in the steel, and further oxide inclusions are included. Therefore, even if the total [O] concentration is small, the life of Al ₂ O ₃ -based inclusions of 20 μm or more becomes remarkably short. However, this is because, according to the studies by the present inventors, there is a problem in the deoxidizing method, that is, the method of adding Al to the molten steel, for example, in the total [O] concentration in the steel or in the slag. (FeO), (MnO),
When Al is added at a high (SiO ₂ ) concentration,
The amount of Al ₂ O ₃ increases, and consequently the total [O] concentration in the steel also becomes relatively high. Further, in this case, the amount of Al ₂ O ₃ of 20 μm or more also increases in the steel. An object of the present invention is to overcome the above-mentioned problems of the above-mentioned conventional techniques and to establish a technique for refining a bearing steel having few inclusions and long rolling fatigue life.

[0005]

In order to achieve the above object, it was found that it is effective to switch the deoxidizing method according to the [O] level of the molten steel in the refining stage, and the method of the present invention was conceived. .. That is, the present invention is C: 0.80 to 1.
Crude molten steel containing 20 wt% is first subjected to preliminary deoxidation by [C] in a vacuum refining device, and then Al is added to the preliminary deoxidized molten steel to deoxidize it. It is a method of refining molten steel.

The composition of the bearing steel obtained by carrying out the refining method of the present invention has a composition of C: 0.80 to 1.20 wt% and Si: 0.
10 to 0.60 wt%, Mn: 0.20 to 0.80 wt%, Cr: 0.30 to 2.0 wt
%, Mo: 0.50 wt% or less, Al: 0.007 to 0.050 wt%, O:
It contains 10ppm or less, the balance is Fe and unavoidable impurities, and among the inclusions investigated by the product round bar, the particle size is
It is steel in which the amount of Al ₂ O ₃ inclusions of 20 μm or more is less than 1/100 mm ² .

[0007]

Operation: Fig. 2 shows C: 1.0 wt%, Si: 0.25 wt%, Mn: 0.
It is a figure which shows [C]-[O] equilibrium of the steel for bearings which has a chemical composition of 40 wt%, Cr: 1.35 wt%, Mo: 0.02 wt%.
The free [O] concentration in equilibrium with the molten steel having the above composition of [C] = 1.0% is about 50 ppm at Pco = 760 torr.
It is less than 1 ppm at Pco = 10 torr. Therefore, as can be seen from this figure, when vacuum deoxidation by [C] is performed,
An extremely low free [O] concentration steel can theoretically be achieved, and when Al is added to such a low free [O] concentration molten steel, the number of particles of Al ₂ O ₃ produced in the molten steel decreases. And large Al ₂ O with a particle size of 20 μm or more
It is expected that the production of ₃ particles will be significantly reduced.

The free [O] concentration and the concentration of (FeO + MnO) in the ladle slag when Al is added to make the Al ₂ O ₃ particle diameter less than 20 μm are 20 ppm or less and 1.0 wt, respectively.
It is desirable to maintain the percentage below. The reason is that in the former case, when the free [O] concentration exceeds 20 ppm, the Al ₂ O ₃ particles after Al deoxidation increase, so that the amount of Al ₂ O _{3 of} 20 μm or more tends to increase. On the other hand, in the latter case, if the (FeO + MnO) concentration in the ladle slag exceeds 1.0 wt%,
This is because (FeO + MnO) reacts with Al in the molten steel to produce Al ₂ O ₃ .

According to the research conducted by the present inventors, Al ₂ O ₃
Particle size is less than 20 μm and the number is 100 mm ²
It has also been found that the rolling fatigue life of the conventional bearing steel is improved 10 times or more when the number is 0.5 or less, more preferably 0.2 or less.

[0010]

[Example] Decarburization refining was carried out in a converter using low-P and low-S blast furnace hot metal which had been subjected to hot metal pretreatment. During this refining, in order to manufacture bearing steel, alloys such as Si, Mn and Cr were added in the converter and / or in the ladle after tapping to adjust the composition. In this refining, the amount of converter slag flowing out into the ladle was reduced as much as possible, and a synthetic flux was added into the refining ladle. The tapping amount is 200t, tapping temperature is
The temperature was 1640 ° C., and the amount of the synthetic flux added was 2.5 t. The composition of this synthetic flux is CaO 80%-
CaF _{2 of} 20% was used. The refractory material of the ladle was high alumina.

The crude molten steel obtained by the above refining was vacuum-degassed using a reflux type vacuum degasser. The vacuum degree during the treatment was 2 torr or less, the treatment time was 30 min, and the molten steel temperature after the treatment was 1600 ° C. Subsequently, while heating the molten steel using an electrode heating type refining furnace (LF apparatus), Ar gas was blown from a porous plug installed at the bottom of the ladle to stir the molten steel. The processing time was 50 min. At this time, shot of Al was added on the molten ladle slag, and the molten Al in the slag was gradually diffused into the molten steel to obtain a desired [Al] concentration. The free [O] concentration before adding Al was 15 ppm or less. Before adding Al, add (Fe
The total concentration of O) and (MnO) was 0.6% or less, and the concentration of (SiO ₂ ) was 10% or less.

On the other hand, as a comparative example, instead of deoxidation by [O] under vacuum, refining with the above synthetic flux using an LF apparatus was first carried out, followed by vacuum degassing under reflux. Using the apparatus, Al was added to the molten steel in the vacuum tank and vacuum degassing treatment was performed. The free [O] concentration before the addition of Al during this comparative refining was 50 to 100 ppm.

The composition of the obtained steel of the conformity example of the present invention is as follows: C = 1.0 wt%, Si = 0.27 wt%, Mn = 0.40 wt%, Cr =
1.35wt%, Al = 0.022wt%, P = 0.015wt%, S = 0.00
A product showing 3 wt% and [O] = 5 ppm was obtained. The composition of the steel of the comparative example is as follows: C = 1.0 wt%, Si = 0.25 wt
%, Mn = 0.38wt%, Cr = 1.37wt%, Al = 0.020wt%, P
= 0.018 wt%, S = 0.003 wt%, [O] = 5 ppm, which were almost the same composition.

Next, the above molten steels are continuously cast to 400 mm ×
A 560 mm bloom slab and immediately rolled to 150 mm x 1
A billet having a diameter of 50 mm was formed and further rolled to obtain a round bar having a diameter of 60 mm. The total [O] concentration of the round bars was 5 ppm or less. When a large inclusion having a size of 20 μm or more was examined by a microscope with a cross section of a round bar, it was remarkably small at 0.2 or less in the example when converted per 100 mm ^{2 of the} area to be inspected.
There were as many as three. Then, the rolling fatigue life of these steel samples was examined by a thrust type rolling fatigue tester. As a result, the rolling fatigue life of the example was 10 times or more that of the comparative example.

[0015]

As described above, according to the present invention, Al
Since it is possible to easily purify a low oxygen content molten steel having a very low content of ₂ O ₃ type inclusions, it is possible to manufacture a bearing steel having excellent rolling fatigue life characteristics.

[Brief description of drawings]

FIG. 1 is a graph showing the effect on rolling fatigue life on the total [O] content in steel.

FIG. 2 is an explanatory diagram of [O]-[C] equilibrium of bearing steel.

Claims (1)

[Claims] 1. A crude molten steel containing C: 0.80 to 1.20 wt% is first subjected to preliminary deoxidation by [C] in a vacuum refining apparatus, and then deoxidized by adding Al to the preliminary deoxidized molten steel. A method for refining molten steel for bearing steel, characterized by acidifying.